26 September 2013

Crystal-encased atoms could harvest waste heat to generate power

Every time you tweet about your meal or check out a new LOLcat, your mobile device churns away to crunching bits and flinging data through the air to make it all happen seamlessly. While technology is still advancing at a merciless rate, it’s still not as efficient as it could be. All electronic devices lose some energy to heat — energy that could be put to better use. A team of researchers at Vienna University of Technology (TU Vienna) believe they have developed a new class of thermoelectric crystalline materials that could harvest that heat and turn it into electricity with high efficiency.

All materials have some small thermoelectric effect, but it is vanishingly tiny in most cases. The miniscule number of substances that exhibit useful thermoelectric effects have been studied intensely in hopes they could be used to salvage some of the heat lost in power generation and by various devices. So far, practical options have been few. The TU Vienna team, led by Professor Silke B├╝hler-Paschen has essentially taken two substances of interest in thermoelectric research and combined them to improve the properties of the resulting material.

The exterior of the novel material is composed of a clathrate compound, a type of crystal with a lattice that can trap small molecules or individual atoms. In this case, the clathrate was used to encase atoms of cerium (atomic number 58). The magnetic properties of cerium were found to boost the thermoelectric capacity of the crystals by more than 50%. That means a much higher voltage can be extracted from the same amount of waste heat.

This material, like all thermoelectric materials, relies on the basic premise of connecting something cool to something warm. Electrons move more rapidly on the warm side, thus there is a diffusion of electrons to the cool side. So the temperature differential across the material is what drives the generation of current. To harvest it, simply route that current to a battery or directly to some other component.

According to the researchers, clathrates and cerium atoms seem to be the perfect combination to take advantage of waste heat to generate electricity. Clathrates have low thermal conductivity, which is important in a thermoelectric substance because otherwise the temperature could even out across the material. No gradient means no current generation.
The cerium atoms for their part seem to be exhibiting a new high temperature version of the Kondo Effect. This is a quantum mechanical influence on electrons caused by magnetic atoms, but it has previously only been observed at temperatures near absolute zero. The electrons of the cerium atom are linked on a quantum level to those of the clathrate, leading to more electrons diffusing across the temperature gradient.

The TU Vienna researchers are currently investigating ways to make the novel material cheaper by looking for other combinations of rare-earth metals that might have the same properties in clathrates as cerium. There are no moving parts or complicated circuitry with thermoelectrics — they could easily integrated with current technology. Maybe in the future a big chunk of the heat coming from the power amplifier in your smartphone will be converted back to battery juice.

Now read: New breakthrough could bring quantum encryption to smartphones

Research paper: DOI: 10.1038/nmat3756 ”Thermopower enhancement by encapsulating cerium in clathrate cages”


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